This application is related to the following U.S. patents and applications:
The above-listed applications are commonly assigned with the present invention. The above-listed applications and patents are incorporated herein by reference as if reproduced herein in their entirety.
The present invention is directed, in general, to power supplies and, more specifically, to a system and method for improving control loop response of a power supply.
Power supplies are an important and rapidly expanding technology that impacts a broad range of applications including computer systems and telecommunication environments. In many applications requiring a DC output, power supplies employing switched-mode DC-DC converters are frequently employed to advantage. DC-DC converters generally include an inverter, a transformer having a primary winding coupled to the inverter and a rectifier coupled to a secondary winding of the transformer. The inverter generally includes a switching device, such as a field- effect transistor (FET), that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the DC-DC converter for application to a load.
Power supplies are often manufactured and sold as standard power modules, where a variety of different customers may purchase the same standard power module and use it in a variety of end applications. Therefore, the load presented to a given standard power module is application dependent and may comprise a wide variety of impedance characteristics. The characteristics of the load itself and any intermediate elements that couple the load to the power supply directly affect the response of a control loop of the power supply, including system response time and stability. It may, therefore, be difficult to design a single standard power module capable of adequate performance over a wide range of possible applications.
Conventional power supplies are typically controlled using analog control techniques and circuitry. Such analog approaches, whether employing integrated circuits or discrete circuit elements, require a product designer to select a priori certain control loop parameters. Since the various characteristics of a particular load (e.g., capacitance, resistance) may not be known to the designer, the designer generally estimates such characteristics and selects the control loop parameters accordingly. The power supply may be further desensitized to anticipated variations in the characteristics, such as those caused by component tolerances and operating conditions.
Since the exact characteristics of the load are generally unknown, compromises are made in the design of the control loop with the consideration of these variations. System stability and a reasonable response to load transients may thus be achieved. However, the control loop is generally not optimized to the particular load. In addition, the impedance characteristics of the load can change over time, or as a result of the system itself being reconfigured. For example, aluminum electrolytic capacitors are subject to a well documented dryout mechanism in which the capacitance reduces as a function of temperature and time. Such a change in load characteristics would directly affect system performance. As another example, a given standard power module may power one or more boards in a system. As boards are added or removed, the impedance characteristics of the load will change, again affecting system performance.
In addition, the power converter may be subjected to a variety of load current conditions during use. It is well known in many power converter topologies that a control law governing the behavior of the power converter can change as a function of the load current. For example, in buck derived power converters, the control law can change as the power converter transitions through a critical current point (e.g., a change from continuous current mode to discontinuous current mode). Such changes in the control law of the power converter can affect a performance of the system employing the power converter, since a single loop compensation design may not be optimized for both modes of operation.
One way of optimizing the control loop is to exhaustively characterize a particular load. The control loop may then be adjusted accordingly by changing various circuit elements in the controller that determine the control loop parameters. This method, however, is generally time and cost intensive. Also, in this method, the load would usually be characterized at only one time, usually as the power module enters service. Variations in the load impedance subsequent to the initial characterization would not be accounted for, again affecting system performance.
Accordingly, what is needed in the art is a system and method for improving response of a control loop of the power supply that overcomes the deficiencies of the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a system and method for improving response of a control loop of a power supply. The power supply is configured to drive a load having at least one characteristic associated therewith. In one embodiment, the system includes: (1) a sensing circuit, associated with the power supply, configured to sense the characteristic; and (2) a compensation circuit, coupled to the sensing circuit, configured to adaptively adjust the control loop based on the characteristic to improve the response.
The present invention introduces, in one aspect, a system for improving the response of a control loop of a power supply based on certain characteristics of a load coupled to the power supply. A load may present a range of variable characteristics that directly affect the response of the power supply control loop, such as system response time and stability. The present invention advantageously senses at least one of the characteristics, allowing the control loop to be adaptively adjusted to improve the response of the power supply.
In one embodiment of the present invention, the system forms a portion of a controller of the power supply. The controller is capable of operating the power supply to provide a current to the load. The sensing circuit then monitors a voltage of the load and determines the characteristic as a function of the voltage. In a related embodiment, the sensing circuit determines the characteristic as a function of a change in the voltage. In another related embodiment, the power supply provides the current to the load during a startup period. The amount of load capacitance, inductance, or other characteristics may be inferred, allowing appropriate adjustments to be made to the control loop.
In a related embodiment, the current is selected from the group consisting of: (1) a constant current; (2) a pulsed current; (3) a ramp current; and (4) a current having a periodic waveform. The current allows the characteristic to be readily sensed. Of course, in place of a current, a voltage may alternatively be supplied. The sensing circuit then determines the characteristic as a function of a change in sensed current.
In one embodiment of the present invention, the system forms a portion of a controller that provides a drive signal having a high frequency signal embedded therein to a power switch of the power supply. The sensing circuit then monitors an output voltage of the power supply and determines the characteristic as a function thereof. The response of the power supply and the load to the perturbation may be sensed by employing, for example, a coherent detection process.
In a related embodiment, the high frequency signal is periodic. In another related embodiment, the high frequency signal is selected from the group consisting of: (1) a sinusoidal signal; and (2) a square wave signal. Of course, other types of signals may be employed as may be appropriate in a particular case.
In one embodiment of the present invention, the characteristic is selected from the group consisting of: (1) a capacitance of the load; (2) a resistance of the load; and (3) an inductance of the load. Of course, other characteristics of the load may be sensed and employed to improve the response of the control loop.
In one embodiment of the present invention, the system forms a portion of a controller that employs a digital integrated circuit, such as a digital signal processor (DSP) or a programmable microprocessor. Of course, general purpose processors may also be employed and remain well within the scope of the present invention.
In one embodiment, the compensation circuit includes a look up table containing loop coefficients. In an alternative embodiment, the compensation circuit employs an algorithm that calculates loop coefficients. In either case, the compensation circuit employs the loop coefficients to adjust the control loop.
In one embodiment, the compensation circuit adaptively adjusts the control loop during a startup period. The control loop may thus be optimized for a particular load.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.